The Commodore 1541 was the most popular disk drive manufactured for the Commodore 64. It was the successor to the 1540 drive (intended for the VIC-20) and actually was a small computer in itself. The DOS (Disk Operating System) resides inside the 1541 and it actually contains a MOS 6502 processor (essentially the same processor inside the C64 itself).
The disk drive used Group Code Recording (GCR). The number of sectors per track varied from 17 to 21 (an early implementation of Zone Bit Recording). The drive's built-in disk operating system was CBM DOS 2.6.
The drive has been criticized as being extremely slow, for reasons that are explained in the book On the Edge: the Spectacular Rise and Fall of Commodore by Brian Bagnall.
The capacity of the drive is 170 KB per side and it was common for the drive to go out of alignment. The speed problem was partially addressed by a bunch of different "fast load" cartridges and software solutions, with the Epyx Fast Load being the most popular.